Figure 4.4

The Nitrogen Cycle

• While N2 is the ultimate source and sink of biospheric nitrogen, several oxidized and reduced forms occur in the environment.

Nitrogen Fixation

Figure 15.17

• Nitrogen-fixing bacteria may be free-living in soil or water, or they may form symbiotic associations with plants.

• - Rhizobium and legumes

The Nitrogen Cycle

Figure 4.5

FeMoCo

• The nitrogenase complex includes two kinds of subunits:

The Mechanism of Nitrogen Fixation

- A protein with an iron-sulfur core (Fe protein) - A protein containing a complex of molybdenum, iron, and sulfur protein (FeMo protein)

Electrons acquired by Fe protein (with energy from ATP) are transferred to FeMo protein to reduce nitrogen. Figure 15.19A

Figure 15.20

Figure 15.21B

• Nitrogen gas (N2) is fixed into ammonium ion (NH4

+) only by some species of bacteria and archaea.

• Aquatic cyanobacteria develop special cells called heterocysts to fix N2.

• - Photosynthesis is turned off to maintain anaerobic conditions.

Nitrogen Fixation

Figure 15.18

Figure 21.33